1. Technical Field
The present invention relates to portable electronic devices in general, and in particular to a method for supplying electric power from a common battery system to a system device and a calendar timekeeping circuit, and for keeping the timekeeping circuit functional as long as possible as the system device's remaining battery charge decreases.
2. Description of Related Art
In a computer system, a semiconductor chip called a Real Time Clock (RTC) chip is typically utilized to provide calendar time to the computer system. In a portable computer, a system battery can be utilized to power the RTC chip. However, the system battery may not always be available—it may be completely discharged or not installed. Hence, the RTC chip is also powered by a dedicated backup battery, generally an RTC coin battery mounted on the system motherboard.
The backup battery also powers an RTC memory, which is used to store the system's calendar time, wakeup time, BIOS setup data, and the like. The RTC chip performs a periodic timekeeping operation to update the stored calendar time. The calendar time in the RTC memory is initialized by a user or in synchronization with a Network Time Protocol (NTP) server.
The RTC memory is usually a volatile memory such as an SRAM. So, if mistaken setup data renders the computer unbootable, the battery system and the RTC coin battery can be removed to erase data stored in the RTC memory. The RTC coin battery cannot be removed unless the system housing is opened, which prevents the user from erasing time information or setup data by mistake.
Previous inventions disclose operating an RTC chip with a main battery and no coin cell. Data that is meant to be non-volatile may be saved into a non-volatile storage when the voltage across the main battery has dropped to a predetermined value. However, without the coin cell and when the system battery is not user-replaceable, the user cannot disable power to the RTC chip to reset the RTC memory. Further, when the system and RTC chip use a common battery system, the need to power the RTC chip should not affect system battery life.
In addition, even when the remaining capacity of the battery system has dropped, there is a need to supply electric power to the RTC chip as long as possible. Also, it is desired that the RTC circuit receiving the supply of electric power from the common battery system should keep its function as much as it can in the case of being supplied with electric power from the RTC coin battery.